Optical measuring apparatus for measuring objects on machines

ABSTRACT

The present invention teaches a method and apparatus for making measurement of an object on a machine, such as a machine tool, using an optical measuring apparatus which includes a light source for generating a beam of light which is incident on a detector. A detection signal is generated within the detector each time the beam is interrupted. The duration and/or frequency of the detection signals are evaluated and an output signal is emitted from the detection only if a further detection signal is present within the detector in a specified time interval from the generation of an earlier detection signal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical measuring apparatus whichenables a coordinate positioning machine (such as a machine tool) todetermine the position of an object relative to a reference point. Itmay, for example, be employed on a machine tool for toolsettingoperations.

2. Description of Related Art

A known tool setting device for use on a machine tool includes a lightsource which generates a fine beam of light which is incident upon adetector. During a toolsetting operation, the machine is operated tomove the tool in a direction transverse to the direction of propagationof the light beam until a part of the tool interrupts passage of thelight beam. Detection of this interruption is used to produce a triggersignal in the detecting unit, which is used by the machine to establishthe relative position of its moving parts in order to determinedimensions of the tool. Such devices are known, for example, from GermanPatent Nos. DE 42 385 04 and DE 42 448 69, French Patent No. 2,343,555,European Patent No. 98,930 and U.S. Pat. No. 4,518,257. The devices maybe used additionally for measuring the length or diameter of a tool tomonitor tool breakage or wear.

The devices disclosed in the above-mentioned patent specifications use anarrow light beam into or through which the tool is passed. Thedetection units detect when the tool breaks into the beam from theresulting drop in the intensity of the light falling on them. Thetrigger signal may be produced as a result of a predetermined drop inthe intensity of light falling on the detector as the tool enters thebeam.

A problem which arises with such optical measuring apparatus is thatcoolant used on the machine can drip through the beam, or be thrown offthe rotating tool into the beam, during the measuring operation and giverise to false trigger signals.

One method of overcoming this problem which is currently used, is toprogram the software in the machine controller to perform severalmeasurements until a pre-selected number of measurements falling withina given tolerance have been obtained. The position of the tool is thenassumed to be the average of these measurements. This method can giverise to an unacceptable increase in the measurement cycle time if asignificant number of repeat measurements have to be taken.

SUMMARY OF THE INVENTION

The present invention seeks to alleviate this problem by providing amethod of measurement which can differentiate between a genuine tooldetection signal, and a signal produced by a coolant drip.

According to one aspect of the present invention there is provided amethod of making measurements of an object on a machine tool using anoptical measuring apparatus which includes a light source whichgenerates a beam of light which is incident upon a detector, the methodcomprising the steps of:

generating a detection signal within the detector each time the beam isinterrupted;

evaluating the frequency and/or duration of the occurrences of saiddetection signals;

emitting an output signal from the detector only if a further detectionsignal is also present within the detector in a specified time intervalfrom the generation of an earlier detection signal.

The timing of the detection signals can be achieved in various ways.

In one embodiment of the invention the tool is rotated, preferably, at aknown specific speed. This gives rise to the generation of a regularsequence of said signals within the detector as the cutting edge (oredges) of the tool interrupt the beam. The generation of the first oneof the said signals is used to initiate a timing sequence within thedetector which sets a time interval (t₁) substantially equal to the timetaken for one revolution of the tool, followed by a second time interval(t₂) which is substantially shorter than (t₁). If it is the tool whichhas generated the signals in the detector then a second signal will begenerated in the time interval (t₂) as the cutting edge of the toolcomes round again, and if this happens, the detector emits the outputsignal.

Alternatively, the generation of a detection signal within the detectorcan be used to start a clock which emits pulses of short durationsynchronised with the speed of rotation of the tool. Again, if a seconddetection signal is generated within the detector during such a pulsethen the detector emits an output signal. A number of clocks may be usedwhich start sequentially as the detector generates its detectionsignals, and each of which stops if no second detection signal isgenerated within the detector during the next one of its pulses.

The invention also includes optical measuring apparatus for carrying outthe method which comprises a light source for generating a light beamand a detector for receiving said beam and which generates a signal whenthe beam is interrupted, wherein the detector includes a detectioncircuit which generates a detection signal each time the beam isinterrupted, and signal processing means for evaluating the frequencyand/or duration of the occurrences of said detection signals and whichemits an output signal only if a second signal is generated by thedetection circuit within a specified time interval after the occurrenceof an earlier detection signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described, by way of example only,with reference to the accompanying drawings in which:

FIG. 1 is a perspective view of an optical measuring apparatusincorporating the present invention;

FIG. 2 is a representation of the output of the detector of FIG. 1; and

FIG. 3 is a block diagram representing the basic elements of theapparatus;

FIG. 4 illustrates in lines 4 a to 4 f the signals generated in variousparts of the signal processing circuit of the detector.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, the optical measuring apparatus is shown in aset up arranged to operate as a toolsetting apparatus, suitable for use,for example, on a machine tool. The apparatus includes a light emittingunit 10 which emits a beam 12 of light, and a light detecting unit 14,where the light beam 12 is detected. Power and signal control cables tothe light emitting and detecting units 10,14 are routed via inlet ports16, and both the units 10,14 are advantageously mounted, via pillars 18,on the base of the machine, either via an intermediate base 20, to whichthey are both mounted, or directly to the base of the machine upon whichthey are to be employed.

In operation, the apparatus is used for toolsetting by operating themachine on which the apparatus is mounted to move the tool in adirection transverse to the direction in which the beam 12 ispropagating. When a predetermined level of occlusion of the beam hasbeen established, the detecting unit 14 emits a trigger signal which isused by the machine to determine the relative position of its relativelymovable parts, thereby to enable dimensions of the tool to bedetermined.

Further mechanical and optical details of an example of such anapparatus are described in European Patent Application No. 00303749.6.

FIG. 2 shows the output of the detector in various circumstances. Thedetector output goes high (i.e. produces a detection signal) as shown bya voltage pulse when the beam is occluded to said predetermined extent.As can be seen by the first pulse S1 at the left-hand side of thedrawing this can occur when a coolant drip passes through the beam.

In the first instance however, the coolant drip is a single occurrence,which produces a single short duration pulse.

When the edge of the rotating cutting tool breaks the beam there is alsoa short duration pulse S2, but this is followed by further pulses S3(only one shown) as the same cutting edge comes into the beam again, oras other cutting edges of a multi-edged tool cut the beam in turn.

In order to be able to identify the difference between occlusion of thebeam by a drip, and occlusion of the beam by the edge of the cuttingtool for the first time (which is the event which is required to bedetected to measure the position of the cutting edge), the inventionprovides that a timer in the detector sets a first time interval t₁simultaneously with the detector generating its detection signal. In aspecific embodiment the time interval t₁ is arranged to be equal to thelength of time it takes for one revolution of the tool. At the end ofthe time interval t₁ the timer sets a shorter time interval t₂.

The detector monitors the time interval t₂ for a second detection signalwhich is unlikely to occur if the first signal is a drip. This can bedetected by either a high state of the output signal or a rising edge.If the second detection signal is present the detector issues a “skip”or trigger signal at the end of time interval t₂.

Since the time intervals t₁ and t₂ are accurately known the instant ofthe occurrence of the first rising edge of the detector output due to acutting edge of the tool obscuring the beam can be calculated.

In order to calculate the timings, the speed of rotation of the machinespindle, and hence the cutting tool must be set. In order to keep thetime for the measuring operation down a reasonable level, the spindlespeed was set during experimentation at 1000 rpm so that t₁ wasostensibly 60 ms. The interval t₂ however, has to be large enough tocater for slight variations in spindle speed for example up to 5%, whichwould cause a 3 ms variation in t₁.

To center the trigger signal in the time interval t₂, t₂ is set at t₁+½twhich must be equal to 60 ms, where t is the variation in t₁ or 3 ms.Thus t₁ was actually set at 58.5 ms.

It is not necessary for the speed of rotation of the tool to be knownprior to measurements being taken since it can be measured by timing thedistance between the rising edges of the first two consecutive pulsesout of the sequence of pulses generated by the detector. The timeinterval (t₁) can then be set between the second and third rising edgesand (t₂) can be timed from the third rising edge.

The basic elements of the apparatus of the invention are shown in blockdiagram form in FIG. 3. The light beam 12 from the transmitter 10entering the detector 14 impinges on a photodetector in a detectioncircuit 72 which produces the signals when the beam is interrupted.Signals generated in the detection circuit 72 are passed to a signalprocessor 74 which includes the necessary timing devices for signalanalysis. The detector output signals are passed directly to the machinecontroller 80 which stops the machine, and evaluates the machine scalereadings to determine the position of the machine.

FIG. 4 illustrates an alternative embodiment in which one or more clocksin the detector are used, each of which generates a series of pulsesinitiated by the detector issuing a signal to indicate that the beam hasbeen interrupted;

FIG. 4a shows an example of a series of events created by a mixture ofteeth and drips interrupting the beam;

FIG. 4b shows the signals created by these events converted to pulses atthe output of a comparator in the signal processor of the detector;

FIG. 4c shows the situation which occurs in the detector when a singleclock is used;

FIG. 4d shows the situation which occurs in the detector when a secondclock is used;

FIGS. 4e and 4 f respectively show the results of combining thecomparator output with the outputs of clock 1 and clock 2.

It can be seen from FIG. 4a that the drips occur and interrupt the beamat random intervals while the beam interruptions caused by the edge ofthe tool occur at regular intervals. Each beam interruption is labelledas a numbered event E.

FIG. 4b shows the comparator output pulses corresponding to the events.

FIG. 4c shows that the first clock starts when event E1 occurs butbecause there is no event occurring when the second clock pulse is sentto the signal processor the clock is stopped. The clock is started againon the occurrence of event E3 which is also a drip, but again becauseits second pulse occurs between events E4 and E5 it will not see eventE4 and will stop. The situation is the same when it starts again atevent E5. Only when it starts again at event E7 will its pulses besynchronised with the occurrences of the cutting edge of the toolinterrupting the beam at event E9 and beyond so that a trigger signalwill be produced on event E7. This signal will have missed the firstoccurrence of the cutting edge interrupting the beam at E6 and willproduce an erroneous reading.

With the embodiment which uses two clocks however, as shown in FIG. 4d,the second clock will be started on event E6 because at that time thefirst clock is running. Since event E6 is an interruption of the beam byan edge of the cutting tool there will be a further event E7 occurringwhen the clock pulse is generated. The signal processor will recognisethat the clock pulses and the events have now become synchronised andwill generate a trigger signal on the rising edge of the pulse. Sincethe generation of the clock pulses and the beam interruptions aresynchronised, the time at which the first beam interruption occurred caneasily be determined.

The situation is less complex if the first event is caused by a cuttingedge of the tool, because the first clock will start and will see asynchronised event during its first pulse and cause a trigger signal atthat time.

If a drip occurs between the first and second events this will beignored by the first clock since it will not occur during its firstpulse. Hence the drip will not affect the generation of the triggersignal under these circumstances.

Although the invention has been described using one or two clocks, otherbenefits may be achieved if more clocks were used which are set atdifferent frequencies.

For example, the apparatus could be used at different speeds of spindlerotation without having to re-set the timing of the existing clocks, andadditional clocks would allow the apparatus to deal with an atmospherewhere a lot of drips could be expected. The number of clocks used wouldbe a trade-off between the benefits to be obtained, and the expense ofthe additional signal processing capability required.

The invention can also be used when the tool is not rotating to measuretool length or diameter during tool setting, or for tool breakagedetection. In such an embodiment the tool is moved at right angles tothe beam until its tip or flank interrupts the beam. The signal producedby the detector is used to start a clock in the signal processor whichevaluates the detector output after a time t. If the detector output isstill high at that time, denoting a signal is still present, the signalprocessor produces a trigger signal.

In yet another embodiment, the signal processor may incorporate a devicewhich identifies synchronous events. The inputs to the device aresamples at regular intervals and the samples are stored in a buffer offixed length, the new contents constantly overriding the old. The buffercan be implemented using a shift register which monitors the detectoroutput and writes its current state into the shift register each time asample is taken. If the buffer is split across two bytes, testing forrepetitive patterns can be achieved by comparing the two halves. Forexample, if the sample rate is eight times the speed of rotation of thetool and the result of each sample is shifted through two eight bitregisters, when a first signal occurs this shows up as a high (1) in thefirst cell of the register. The sample moves through the register untilit is passed into the first cell on the second half. If there has beenno synchronous event the sample passes through the register to the end.

If however, another high sample is received in the first cell of thefirst half of the register just as the first sample moves into the firstcell of the second half of the register then the two halves will becomeidentical once again and a trigger signal will be issued.

The invention has been described with reference to the elimination ofspurious trigger signals in an optical measuring apparatus on a machinetool, but may have wider application using other forms of opticalmeasuring apparatus on other types of machine. The scope of theinvention is therefore, to be taken as that defined by the appendedclaims.

What is claimed is:
 1. A method of making measurements of an objectusing an optical measuring apparatus which includes a light source whichgenerates a beam of light, and a detector, upon which is incident thebeam of light, the method comprising the steps, in any suitable order,of: causing a beam of light to be emitted from the light source;generating a first detection signal within the detector when the beam oflight from the light source is interrupted; providing a first timeinterval when the first detection signal is generated; providing asecond time interval wherein the second time interval is shorter thanthe first time interval and commences at the end of the first timeinterval; emitting an output signal from the detector if a furtherdetection signal is present within the detector during the second timeinterval.
 2. A method of making measurements of an object as claimed inclaim 1 further comprising the steps of: rotating the object; andwherein the first time interval is dependent on the speed of rotation ofthe object.
 3. A method of making measurements of an object as claimedin claim 2 wherein the object is a tool having a tool edge, and thefirst time interval is substantially the time for the edge of the toolto come around again following its interruption with the beam.
 4. Amethod according to claim 3 wherein the apparatus further includes aclock, the method comprising the further steps of: rotating the tool;causing the clock to initiate the emission of a series of pulses each ofwhich are equal in duration to the second time interval and aresynchronised with the speed of rotation of the tool, the first pulsebeing emitted to coincide with a detection signal being generated in thedetector; emitting an output signal from the detector only if adetection signal is also present within the detector during theexistence of a clock pulse; stopping the clock if no such detectionsignal is present in the detector.
 5. A method according to claim 4wherein a clock pulse is produced for each revolution of the tool, andan output signal is emitted from the detector only if a detection signalis present with the detector during the next pulse emitted by the clockfollowing said first pulse.
 6. A method according to claim 4 wherein theapparatus includes two clocks the method comprising the further stepsof: causing a first clock to initiate the emission of a first series ofsaid pulses when a detection signal is generated within the detector;causing the second clock to initiate the emission of a second series ofsaid pulses commencing with the generation of a further detection signalwithin the detector in the interval between two successive pulses of thefirst clock; and emitting an output signal from the detector if adetection signal is also present within the detector during theexistence of the next pulse in the second series if the detector has notemitted an output signal based on the first series of pulses.
 7. Amethod according to claim 6 wherein the apparatus includes additionalclocks, the method comprising the steps of sequentially initiating theemission of respective series of said pulses if a detection signal isgenerated within the detector and all of the previously started clocksare running.
 8. A method according to claim 6 wherein the apparatusincludes additional clocks, the method comprising the steps of settingthe clocks to produce respective series' of pulses at differentfrequencies set to coincide with different speeds of rotation of thetool, and causing the initiation of the emission of a said series ofpulses appropriate to the speed of rotation of the tool.
 9. Opticalapparatus for measuring objects comprising a light source for generatinga beam of light and comprising a detector arranged to receive the saidbeam and for generating a signal when the beam is interrupted, whereinthe detector in use: generates a first detection signal within thedetector when the beam of light from the light source, incident on thedetector is interrupted; provides a first time interval when the firstdetection signal is generated, provides a second time interval whereinthe second time interval is shorter than the first time interval andcommences at the end of the first time interval; and emits an outputsignal from the detector if a further detection signal is present withinthe detector during the second time interval.
 10. Optical apparatusaccording to claim 9, wherein the object is a cutting tool.
 11. A methodof making measurements of an object on a machine using an opticalmeasuring apparatus which includes a light source which generates a beamof light which is incident upon a detector and a detector, the methodcomprising upon a detector and a detector, the method comprising thesteps, in any suitable order, of: rotating the object; causing light tobe emitted from the light source; generating a first detection signalwithin the apparatus when the beam of light from the light source,incident on the detector, is interrupted; generating a first timeinterval when the first detection signal is generated; evaluatingwhether there is a further occurrence of the detection signal during asecond time interval, which commences at the end of the first timeinterval and is shorter than the first time interval; and emitting anoutput signal from the apparatus only if a further detection signal ispresent within the apparatus during the second time interval.